FIELD OF THE INVENTION
The prior art has proposed the construction of temperature or pressure responsive control units which can be adjusted to respond to sensed condition levels anywhere within a relatively wide range of condition levels. Temperature responsive control units of this kind are particularly useful as replacements for malfunctioning original equipment control units in various types of heating or cooling equipment. The equipment serviceman can install and adjust a multiple purpose control unit to duplicate the operation of an original control unit without having to order a particular replacement unit or maintain an inventory of a large number of replacements for original control units. The availability of multiple purpose control units has tended to reduce maintenance expenses and the extent of the down time of heating or cooling equipment caused by original control unit malfunctions.
Prior art temperature controls of the character referred to have been required to operate reliably across relatively wide temperature ranges (for example, from around -20.degree. F. to 100.degree. F. or more). Moreover, they have frequently had to provide the capability of adjusting operating temperature differentials. For example, where a control is provided for a food freezer, refrigeration system operation may be initiated when the sensed freezer compartment temperature rises to 10.degree. F. and terminated when the sensed temperature is reduced to -20.degree. F. The same control, if installed in a beverage cooler, would be required to initiate refrigeration system operation at a sensed temperature of about 40.degree. F. and terminate the refrigeration system operation at around 32.degree. F. The identical control might be installed to govern operation of a heater unit which is turned on at 68.degree. F. and turned off at 72.degree. F. In short, these multiple purpose control units have had to be capable of use in various enviornments having widely differing high and low temperature "event" requirements depending upon the application involved.
The prior art control units have had several disadvantages. In the first place in order to provide necessary versatility the construction of these devices has been relatively complicated. Some units have employed range and differential adjusting mechanisms having multiple springs which must coact consistantly through the operating range of the control and provide for accurately adjustable control "event" condition levels in order for the control unit to operate satisfactorily. Fabrication, assembly and calibration of such units has been difficult and resulted in relatively significant unit costs while the complexity of the construction has tended toward unreliable performance.
Some prior art control units have included a control switch, a condition responsive actuator and a lever for transmitting actuator forces to the control switch. In addition these control units have been provided with various mechanisms by which the response of the units to sensed conditions can be adjusted. In most prior art controls of this character the switch operating levers have been acted upon by relatively heavy spring and actuator forces which must be delicately balanced to assure proper control operation. Separate, relatively complex mechanisms for range and differential adjustments have been proposed, all of which tended to reduce the accuracy, reliability and ease of adjusting the controls while increasing the difficulty of their manufacture and their cost. In one proposal, for example, the switch operating lever has a resiliently shiftable pivot axis whose location is governed according to the interactions of multiple springs associated with complicated adjusting mechanisms.